1. Field of the Invention
This invention relates to zinc phosphate coatings for metal surfaces and a process for phosphatizing a metal surface with acidic aqueous phosphate solution. The invention is applicable to a variety of substrates including cold rolled steel (CRS), zinc alloys and aluminum.
2. Statement of Related Art
Present day phosphate coating solutions are dilute aqueous solution of phosphoric acid and other chemicals which, when applied to the surface of a metal react with the metal surface forming an integral layer on the surface of the metal of a substantially insoluble phosphate coating, amorphous or crystalline. Generally the crystalline coatings are preferred.
Typically the solutions include phosphate ions, zinc and other metal ions to provide specific characteristics desired in the final coating. Other ions typically present may be nitrate, nitrite, chlorate, fluoroborate or silicofluoride. A typical phosphating process is comprised of the following sequence:
(1) Cleaning and conditioning PA1 (2) Phosphating and PA1 (3) Post treating.
Rinses are generally employed between each step to prevent any carry over of materials to the next step.
Despite advances in both the composition of the phosphate coating solution and the phosphating process, there is a continued demand for still further improvements in the compositions and processes, to provide more control over the process, to form adequate coating weights, to reduce formation of scale or white spots, reduce environmental impact and safety concerns.
U.S. Pat. No. 4,838,957 describes a zinc phosphating process employing aqueous phosphate solution containing zinc ion, phosphate ion, manganese ion, fluoride ion and a phosphating accelerator. The accelerator may be one or more of (a) nitrate ion, (b) nitrite ion, (c) hydrogen peroxide, (d) m-nitrobenzene sulfonate ion, (e) m-nitrobenzoate ion or (f) p-nitrophenol. Nickel is indicated as an optional ingredient. While morphology of the coating is not discussed, the coatings are primarily a crystalline platelet structure.
In U.S. Pat. No. 4,865,653 phosphate coating solutions are described in which the accelerator employed is hydroxylamine sulfate (HAS) which is employed so as to alter the morphology of the resulting coating from platelet to a columnar and/or nodular structure over a broad range of zinc concentration. While Ni and Mn are generally mentioned as additional ions, there is no description of specific amounts of either in the patent or any specific examples thereof. The patent further describes a relatively large number of other patents which include hydroxylamine sulfate in zinc phosphate solutions, as well as various oxidizing agents including U.S. Pat. Nos. 2,743,204 and 2,298,280.
U.S. Pat. No. 4,793,867 describes a coating composition which includes zinc and another divalent cation, such as manganese or nickel in addition to a non-coating monovalent cation, such as sodium or potassium to provide improved alkaline solubility of conversion coatings applied to zinc-coated substrates. HAS is noted as eliminating any unwanted precipitation which may arise in adding any manganese alkali. Three U.S. Pat. Nos. 4,389,260; 4,486,241 and 4,612,060 are cited in the list of references cited in U.S. Pat. No. 4,793,867. These patents relate to zinc phosphating solutions which contain nickel and/or manganese.